In a fibre optic gyroscope a clockwise (CW) beam and a counter-clockwise (CCW) beam of light pass round a coil of optical fibre and are brought together on leaving the coil to interfere on a detector. When rotation is applied to the gyroscope a rate-related non-reciprocal phase shift is introduced between the CCW and CCW beams which varies the intensity on the detector.
With such fibre optic gyroscopes there is a need for accuracy and improving this accuracy often entails a considerable increase in cost. One way of achieving good accuracy has been by the use of a conventional closed loop fibre optic gyroscope which has the disadvantage of using expensive lithium niobate integrating optic circuits in the optics to give the required broadband phase modulation which is generally in two parts. Whilst this type of gyroscope gives a good performance over a high rate range it is relatively complex, requires precision modulators and is thus relatively expensive to manufacture.